We propose to continue our studies examining the uptake of radioactive substrates in isolated perfused fetal, newborn and adult rabbit lungs. Our experimental rationale is simply to confirm previously published uptake studies in our young adult rabbit lung system. Then the uptakes during the stages of development represented by the 26-day fetal and the newborn rabbit lung preparations will be measured relative to the observed adult uptake. The uptake of H3-norepinephrine will be completed during the next year's work and another substrate will be begun. Some choices being considered include cortisone, bradykinin or angiotensin I.